Flu outbreak defies the trend for infection-free cruising.

According to the US Centres for Disease Control, 561 passengers were treated earlier this month for stomach flu over the course of a cruise from Port Canaveral, which was shortened from seven days to six because of the outbreak.

Health regulators in the United States are still investigating the early January outbreak of gastrointestinal illness on the Oasis of the Seas, one of the largest on a cruise ship in the past 10 years.

Paradoxically, it arrived on the heels of 2018, which saw the lowest number of onboard outbreaks in at least 17 years.

According to the US Centres for Disease Control, 561 passengers were treated earlier this month for stomach flu over the course of a cruise from Port Canaveral, which was shortened from seven days to six because of the outbreak.

The number of those treated amounted to 8.9% of the passengers on the ship. Authorities in Jamaica advised Royal Caribbean International not to disembark passengers on a port call in Falmouth, and the ship's captain made the decision to cancel a call at the next port, Cancun, and return home.

The severity of the attack prompted Royal to issue a full refund to all 6,285 passengers onboard, a rarity for voyages that suffer outbreaks.

Royal Caribbean did not comment on the reasons for issuing a full refund on the Oasis. But in a statement, it said cutting short the cruise was "the right thing to do to get everyone home early rather than have guests worry about their health”.

The massive outbreak on the Oasis is especially confounding in light of the decline in such illness on cruise ships in 2018. Last year, fewer people fell sick in outbreaks recorded on 10 ships than became ill on the single voyage of the Oasis.

Only 547 passengers fell ill in the 2018 outbreaks, the fewest since at least 2002, when a mutation in the virus led to a new strain that sent outbreaks on cruise ships soaring for several years.

Notably, only one of the 10 outbreaks caused more than 100 passengers to get sick last year.

In a statement, the Vessel Sanitation Programme, part of the U.S. Centres for Disease Control, said fewer and less severe outbreaks in 2018 were likely the result of several factors.

One was earlier detection of illness. When sick passengers are spotted by crew members, or if passengers volunteer their illness early in the cruise instead of waiting, procedures for cleaning the vessel and quarantining the ill work better.

Another factor cited by the CDC was "cruise industry diligence" in developing and implementing outbreak prevention and control plans. In 2016, CLIA formulated its "Sample Outbreak Prevention and Response Plan" for members. It was particularly useful for smaller or new cruise lines that hadn't previously adopted protocols.

In recent years, all cruise lines have required passengers to complete a health form upon boarding or acknowledge in some way that they are not suffering symptoms that could be caused by norovirus.

Some cruise lines do not allow passengers to serve themselves from the buffet, at least in the initial days of the cruise.

Hand sanitizers are omnipresent in areas where food is served. In designing new ships, cruise lines are also building hand-washing stations with running water and soap at the entrances to buffet restaurants.

Over the years, cruise lines have also homed in on more effective disinfection and cleaning agents for norovirus and have refined their methods for interior cleaning after an outbreak occurs.

All those responses have contributed to a gradual decline in outbreaks over the past 15 years, the CDC said. With the exception of 2012, the agency said, "The rate of acute gastroenteritis illness on cruise ships has decreased over time, as has the number and severity of outbreaks by year."